1. Field of the Invention
The present invention is concerned with an improved exhaust system used with an internal combustion engine. More specifically, the present invention is concerned with an exhaust system which has at least one catalytic device included in or attached to it for reducing harmful emissions or purifying engine exhaust gases.
2. Description of Related Art
Known exhaust systems for internal combustion engines, which discharge engine exhaust gases into the atmosphere, are typically endowed with catalytic devices which reduce harmful emissions or purify exhaust gases. One such catalytic device and engine arrangement is known from, for example, Japanese Unexamined Patent Publication 63-102,922. In the arrangement described in this publication, the exhaust system has a plurality of catalytic convertors provided for each row of engine cylinders. With regard to the capacity of the catalytic device which is located farthest upstream, i.e., closest to the engine, the lengths of the exhaust passage between each row of air pipes and each catalytic device is arranged so that they vary from the longest to the shortest. When this construction is used, after the engine is started, the temperature of each catalytic device, starting from that catalytic device which is the farthest upstream, is increased until it reaches a gas purifying temperature an operational temperature) without any great delay. At the gas purifying or operational temperature, a specific level of gas purification is demonstrated.
The known exhaust system of the Japanese publication mentioned above, however, needs a long time after operation of the engine starts before purification of exhaust gases begins. In particular, while the engine is cold and until the engine has been adequately heated up, purification of exhaust gases is insufficiently performed.